canImport should be able to take an additional parameter labeled by either version or
underlyingVersion. We need underlyingVersion for clang modules with Swift overlays because they
have separate version numbers. The library users are usually interested in checking the importability
of the underlying clang module instead of its Swift overlay.
Part of rdar://73992299
Not-filtering solutions causes unacceptable slownesses in some cases.
For now, filter solutions as normal typechecking does to restore the
performance.
rdar://76714968
Having purpose attached to the contextual type element makes it much
easier to diagnose contextual mismatches without involving constraint
system state.
This saves us from needing to re-match args to params in CSApply and is also
useful for a forthcoming change migrating code completion in argument position
to use the solver-based typeCheckForCodeCompletion api.
rdar://76581093
Conformance constraints could be transferred through conversions,
but that would also require checking implicit conversions
such as optional and pointer promotions for conformance is the
type itself doesn't conform, for that let's add a special constraint
`TransitivelyConformsTo`.
One can convert from a function value without a global actor to a
similar function type that does have a global actor, but one cannot
remove or change a global actor on a function type.
Not all of the unary operators have `CGFloat` overloads,
so in order to preserve previous behavior (and overall
best solution) with implicit Double<->CGFloat conversion
we need to allow attempting generic operators for such cases.
```swift
let _: CGFloat = -.pi / 2
```
`-` doesn't have `CGFloat` overload (which might be an oversight),
so in order to preserve type-checking behavior solver can't be
allowed to pick `-(Double) -> Double` based on overload of `/`,
the best possible solution would be with `/` as `(CGFloat, CGFloat) -> CGFloat`
and `-` on a `FloatingPoint` protocol.
This makes it possible to pick `CGFloat` function/operator overloads even
in the presence of literals, otherwise non-default literal score gets in
a way and solver ends up producing a lot more solutions with implicit
Double<->CGFloat conversion than it can disambiguate, so it's better to
just preserve the old behavior and pick `CGFloat` concrete overloads when
appropriate.
- Prefer CGFloat -> Double over the other way around to avoid
ambiguities;
- Every new conversion impacts the score by factor of number of
previously applied conversions to make it possible to select
solutions that require the least such conversions.
- Prefer concrete overloads with Double <-> CGFloat conversion
over generic ones.
New name is `isInKeyPathComponent` since it's implemeneted as a
check for presence of `KeyPathComponent` element in the locator
that covers every sub-element contained in a key path component
as well.
